let _keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
console.log(decode(encode('鹅m')))
function encode(input) {
  var output = "";
  var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
  var i = 0;
  // 将Unicode，变成 utf8
  input = _utf8_encode(input); 
  // 将utf8 转成 base64
  while (i < input.length) {
    chr1 = input.charCodeAt(i++);
    chr2 = input.charCodeAt(i++);
    chr3 = input.charCodeAt(i++);
    enc1 = chr1 >> 2;
    enc2 = ((chr1 & 0b11) << 4) | (chr2 >> 4); // --> 0b11 == 3
    enc3 = ((chr2 & 0b1111) << 2) | (chr3 >> 6); // --> 0b111 == 15
    enc4 = chr3 & 0b111111; // --> 0b111 == 63
    if (isNaN(chr2)) {
      enc3 = enc4 = 64; 
    } else if (isNaN(chr3)) {
      enc4 = 64;
    }
    output = output +
      _keyStr.charAt(enc1) + _keyStr.charAt(enc2) +
      _keyStr.charAt(enc3) + _keyStr.charAt(enc4);
  }
  return output;
}
function decode(input) {
  var output = "";
  var chr1, chr2, chr3;
  var enc1, enc2, enc3, enc4;
  var i = 0;
  input = input.replace(/[^A-Za-z0-9\+\/\=]/g, "");
  //将base64转化成utf8
  while (i < input.length) {
    enc1 = _keyStr.indexOf(input.charAt(i++));
    enc2 = _keyStr.indexOf(input.charAt(i++));
    enc3 = _keyStr.indexOf(input.charAt(i++));
    enc4 = _keyStr.indexOf(input.charAt(i++));
    chr1 = (enc1 << 2) | (enc2 >> 4);
    chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
    chr3 = ((enc3 & 3) << 6) | enc4;
    output = output + String.fromCharCode(chr1);
    if (enc3 != 64) {
      output = output + String.fromCharCode(chr2);
    }
    if (enc4 != 64) {
      output = output + String.fromCharCode(chr3);
    }
  }
  // 将utf8转成浏览器认识的Unicode
  output = _utf8_decode(output);
  return output;

}
function _utf8_encode(string) { // 将Unicode，变成 utf8
  string = string.replace(/\r\n/g, "\n");
  var utftext = "";
  for (var n = 0; n < string.length; n++) {
    var c = string.charCodeAt(n); // 获取到uncode编码
    if (c < 128) {// 转化成utf8编码
      utftext += String.fromCharCode(c);
    } else if ((c > 127) && (c < 2048)) {
      utftext += String.fromCharCode((c >> 6) | 192);
      utftext += String.fromCharCode((c & 63) | 128);
    } else { // 转化成utf8汉字
      utftext += String.fromCharCode((c >> 12) | 0b11100000);
      utftext += String.fromCharCode(((c >> 6) & 0b111111) | 0b10000000);
      utftext += String.fromCharCode((c & 0b111111) | 0b10000000);
    }

  }
  return utftext;
}

function _utf8_decode(utftext) { // 将utf8转成Unicode
  var string = "";
  var i = 0;
  var c = c1 = c2 = 0;
  while (i < utftext.length) {
    c = utftext.charCodeAt(i); // 第一个字节
    if (c < 128) {
      string += String.fromCharCode(c);
      i++;
    } else if ((c > 191) && (c < 224)) {
      c2 = utftext.charCodeAt(i + 1);
      string += String.fromCharCode(((c & 31) << 6) | (c2 & 63));
      i += 2;
    } else {
      c2 = utftext.charCodeAt(i + 1); // 第一个字节
      c3 = utftext.charCodeAt(i + 2); // 第二个字节
      string += String.fromCharCode(((c & 0b1111) << 12) | ((c2 & 0b111111) << 6) | (c3 & 0b111111));
      i += 3;
    }
  }
  return string;
}